Injection moulding of plastics is a multi-stage process. The process as such is well known. Often the plastic material is supplied in the form of granules. According to one concept a batch of granules may comprise individual granules having a composition corresponding to the composition of the final product. According to another concept a batch may comprise various types of granules of different compositions such that the batch of granules on a whole has a composition corresponding to the composition of the final product.
The granules are fed to an extruder in which the granules are worked on, usually by means of an extruder screw operating continuously. The extrusion screw may be compared to a transport screw having a varying depth or pitch (or both) to induce an increased amount of frictional work on the granules, allow for trapped gases to be released etc. The design of extruders is a science in itself, and it is well outside the scope of the present specification to provide a detailed description.
The result of the extrusion process may be a melt, and this melt is then fed to an accumulator acting as a buffer between a continuously operating extruder and an intermittently operating injection device. The injection device injects the melt into a mould cavity and the melt sets, or freezes, in the mould cavity. The injection device consequently has a cyclic or intermittent operation in which it injects a melt to a cavity in one stroke, and receives a new melt from the extruder via the accumulator.